Charged hadrons (i.e. protons, pions, ions such as carbon ions) have physical advantages with respect to X-rays or gamma rays in the field of radiation therapy. For example, protons of a given energy (i.e. forming a mono-energetic proton beam), have a certain penetration depth in a target object and do not penetrate beyond that depth, and furthermore, they deposit their maximum amount of energy or dose in the so-called Bragg Peak, which corresponds to said penetration depth, i.e. the point of greatest penetration of the radiation in the target volume. The position of the Bragg peak is also related to the ‘beam range’, which is usually defined as the position where the dose is 80% of the value at the Bragg peak. Since the Bragg peak position depends on the energy of the hadron beam, it is evident that by precisely controlling and modifying the energy, one can place the Bragg Peak at a given depth of a tumour so as to administer the greatest radiation energy to selected points and spare the healthy tissue surrounding said points.
As a consequence, the location of the Bragg peak must be precisely known since critical tissue localized near the target tumour could receive an excessive dose, whereas conversely the target tumour could receive an insufficient dose. There is a need therefore to obtain a direct on-line, i.e. during beam delivery, measurement of the particle range.
One option which has been explored is the detection of prompt gammas emitted from a target irradiated by a charged hadron beam. Prompt gammas are emitted isotropically from every location along the charged hadron beam path in the target, so that this path is seen as a gamma line source by a detection apparatus. The detection of said prompt gammas offers a possibility of determining the beam range. One solution of this type is disclosed in the document ‘Prompt gamma measurements for locating the dose falloff region in the proton therapy’, Chul-Hee Min and Chan Hyeong Kim, 2006 Applied Physics Letters, article 183517. The authors used a gamma scintillation camera equipped with one multilayered collimator system to measure prompt gamma generated by irradiation. Nevertheless, this device is only able to detect prompt gamma emitted from 90° of the beam direction. To obtain the prompt gamma distribution along the beam direction, the detector may be moved step by step to different measurement positions which makes this device not useful for practical on-line measurements.
In the document ‘Development of an array-type prompt gamma detection system for in vivo range verification in proton therapy’, Chul-Hee Min et al, Med. Phys. 39(4), April 2012, pp 2100-2107, a linear array of scintillation detectors and photodiodes is disclosed for the online measurement of the proton beam range. This study discloses a collimator with a plurality of slits, for detection of the beam range in a target. The optimal dimensions of the slits and collimation depths disclosed in this document are as follows: slit width 2 mm, septum width 2 mm, collimator depth 150 mm, pitch 4 mm. These dimensions however do not allow sufficiently high statistics of the detected prompt gammas, as will be explained in the detailed description of the present disclosure.
Further previous studies have explored the possibility of verifying the beam range in a target by detecting shifts of a prompt gamma profile with respect to a reference profile. This approach has been described for the case of a so-called ‘knife-edge’ slit prompt gamma camera in document WO2012104416 and in ‘Prompt gamma imaging with a slit camera for real-time range control in proton therapy’, Smeets et al, Physics in Medicine and Biology, 57(11), 3371-3405.